In a conventional serial data system, symbols may be transmitted sequentially, with the frequency spectrum of each data symbol allowed to occupy the entire bandwidth. A parallel data transmission system is one in which several sequential streams of data may be transmitted simultaneously. In a parallel system, the spectrum of an individual data element may only occupy a small part of the available bandwidth.
In a classic parallel data system, the total signal frequency band may be divided into N overlapping frequency subchannels. Each subchannel may be modulated with a separate symbol. The subchannels may then be multiplexed.
Orthogonal signals may be separated at the receiver by using correlation techniques, eliminating inter-symbol interference. This may be achieved by carefully selecting carrier spacing so as to let the carrier spacing equal the reciprocal of the useful symbol period. Orthogonal Frequency Division Multiplexing (OFDM) is a form of multicarrier modulation wherein carrier spacing may be selected so that each subcarrier is orthogonal to the other subcarriers.
This orthogonality may avoid adjacent channel interference and may prevent the demodulators from seeing frequencies other than their own. The benefits of OFDM may be high spectral efficiency, resiliency to Radio Frequency (RF) interference, and lower multi-path distortion.
In OFDM, the subcarrier pulse used for transmission may be chosen to be rectangular. This has the advantage that the task of pulse forming and modulation may be performed by an Inverse Discrete Fourier Transform (IDFT) which may be implemented very efficiently as an Inverse Fast Fourier Transform (IFFT). Therefore, the receiver may only need a FFT to reverse this operation.
Incoming serial data may first be converted from serial to parallel and grouped into x bits each to form a complex number. The number x may determine the signal constellation of the corresponding subcarrier, such as 16 Quadrature Amplitude Modulation. The complex number may be modulated in a baseband fashion by the IFFT and converted back to serial data for transmission. A guard symbol may be inserted between symbols to avoid inter-symbol interference (ISI) caused by multi-path distortion. The discrete symbols may be converted to analog and low-pass filtered for radio frequency (RF) up-conversion. The receiver then simply performs the inverse process of the transmitter.
According to the theorems of the Fourier Transform the rectangular pulse shape may lead to a sin(x)/x type of spectrum of the subcarriers, as illustrated in FIG. 1. The spectrums of the subcarriers overlap. The reason why the information transmitted over the carriers may be separated is the orthogonality relation. By using an IFFT for modulation, the spacing of the subcarriers may be chosen such that at the frequency where a received signal is evaluated (indicated by letters A-E in FIG. 1) all other signals may be zero.
In a packet communication system, data that is communicated may first be packetized into packets of data, and the data packets, once formed, may then be communicated, sometimes at discrete intervals. Once delivered to a receiving station, the information content of the data may be ascertained by concatenating the information parts of the packets together. Packet communication systems generally make efficient use of communication channels as the communication channels need only to be allocated pursuant to a particular communication session only for the period during which the data packets are communicated. Packet communication channels may sometimes be shared communication channels that are shared by separate sets of communication stations between which separate communication services are concurrently effectuated.
A structured data format is set forth in the present promulgation of the operating specification. The data format of a data packet formed in conformity with standards, such as the ultra-wideband WiMedia or ECMA-368/369, may include a preamble part and a payload part. Other packet communication systems analogously format data into packets that may also include a preamble part and a payload part. The payload part of the packet may contain the information that is to be communicated. That is to say, the payload part may be non-determinative. Conversely, the preamble part of the data packet may not contain the informational content that is to be communicated but, rather, may include determinative data that is used for other purposes. In particular, the preamble part of a WiMedia or ECMA-368/369 packet preamble may include three parts, a packet sync sequence, a frame sync sequence, and a channel estimation sequence. The packet sync sequence and frame sync sequence may be of a length of twenty-four OFDM symbols, and the channel estimation sequence may be of a length of six OFDM symbols. Collectively, the sequences may be of a time length of 9,375 microseconds.
Within the WiMedia PHY layer, implementation may use OFDM as the underlying modulation technique. At its core, it may use 128 unique frequency bins, or “tones,” on which it may modulate information. Of these 128 tones, 6 may be NULL tones that carry no information, 12 may be pilot tones that contain data used for tracking, 100 may be data tones which carry the packet payload and there may be 10 guard tones.
However, there is an ongoing desire to increase OFDM transmission data rates and improving system robustness.